Resistive heater for thermo optic device

ABSTRACT

Resistive heaters formed in two mask counts on a surface of a grating of a thermo optic device thereby eliminating one mask count from prior art manufacturing methods. The resistive heater is comprised of a heater region and a conductive path region formed together in a first mask count from a relatively high resistance material. A conductor formed from a relatively low resistance material is formed directly on the conductive path region in a second mask count. Thermo optic devices formed by these two mask count methods are also described.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.10/231,898, filed Aug. 29, 2002 now U.S. Pat. No. 7,020,365, whichapplication is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to thermo optic devices, such as opticalwaveguides. In particular, it relates to a resistive heater for changingan optical characteristic of the thermo optic device. Even moreparticularly, it relates to an efficiently formed resistive heater.

BACKGROUND OF THE INVENTION

The art of making and developing new uses for thermo optic devicescontinues to emerge. Presently, thermo optic devices are used asfilters, switches, multiplexers, waveguides, and a host of othersemiconductor and optical transmission devices.

With reference to FIGS. 1A and 1B, a thermo optic device in accordancewith the prior art is shown generally as 110. It comprises a cladding115 that includes an upper cladding 114 and a lower cladding 112. A core116 is defined by the cladding and is generally formed of a materialhaving a higher or lower refractive index than that of the cladding. Thecore 116 may, for example, define an optical waveguide, such as aY-shaped optical splitter having an input waveguide 122 and two outputwaveguides 124, 126. The core 116 together with the cladding 115 aresometimes referred to as a grating and are disposed on a substrate 118.The substrate may be formed of silicon but is not required to be. Aheater 120 is disposed adjacent to the cladding 115.

During use, a control element (not shown) delivers current to the heater120, to change an optical characteristic of the thermo optic device. Forexample, in a Bragg grating formed with a polymer grating, when currentis delivered to heater 120, the refractive index of the polymer willchange as a result of the thermo optic effect. In turn, this refractiveindex change affects the wavelength of light that satisfies the knownBragg reflective condition so that a different wavelength is now Braggreflected in the optical waveguide.

If the process is repeated at another temperature, which is a functionof current delivery and heater characteristics, another wavelength willsatisfy the Bragg reflective condition. In this manner, the thermo opticdevice 110 is made tunable. Such a device will normally be operated in asteady state condition so that a single wavelength will satisfy theBragg reflection condition over a given time interval.

With reference to FIGS. 1C and 1D, a portion of the thermo optic device110 is shown in greater detail. In particular, the heater 120 is formedwith contacts 121, 123 and conductors 131, 133 to, ultimately, connectto the control element during use.

Unfortunately, the heater 120, together with its associated contacts121, 123 and conductors 131, 133, requires three fabrication maskingsteps to form with conventional processes, i.e., one masking step toform the heater, one to form the contacts, and one to form theconductors. This unnecessarily complicates manufacturing and wastesresources and finances.

Accordingly, the thermo optic arts desire improved heaters that arecheaper and quicker to produce, e.g., formed by fewer mask counts,without any corresponding sacrifice in quality, reliability orlongevity.

SUMMARY OF THE INVENTION

The above-mentioned and other problems become solved by applying theapparatus and method principles and teachings associated with thehereinafter described resistive heater for thermo optic device.

In one embodiment the resistive heater is formed on a surface of agrating of a thermo optic device in two mask counts thereby eliminatingone mask count from prior art manufacturing methods. In particular, theresistive heater is comprised of a heater region and a conductive pathregion formed together in a first mask count from a relatively highresistance material. A conductor, formed from a relatively lowresistance material, is formed directly on the conductive path region ina second mask count. Advantageously, manufacturing processes can now besimpler and resource waste can be eliminated.

Thermo optic devices formed by these two mask count methods are alsodescribed.

These and other embodiments, aspects, advantages, and features of thepresent invention will be set forth in part in the description whichfollows, and in part will become apparent to those skilled in the art byreference to the following description of the invention and referenceddrawings or by practice of the invention. The aspects, advantages, andfeatures of the invention are realized and attained by means of theinstrumentalities, procedures, and combinations particularly pointed outin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a thermo optic device having a Y-shapedoptical waveguide in accordance with the prior art;

FIG. 1B is a cross sectional view of the thermo optic device of FIG. 1Ataken along line 1B-1B;

FIG. 1C is a more detailed cross sectional view of a heater of thethermo optic device of FIG. 1A;

FIG. 1D is a planar view of the heater of FIG. 1C;

FIG. 2A is a cross sectional view of a grating upon which a resistiveheater in accordance with the teachings of the present invention will beformed;

FIG. 2B is a cross sectional view in accordance with the teachings ofthe present invention of a first layer deposited upon the grating ofFIG. 2A;

FIG. 2C is a cross sectional view in accordance with the teachings ofthe present invention of a first mask used upon the first layer of FIG.2B;

FIG. 2D is a cross sectional view in accordance with the teachings ofthe present invention of a patterned first layer formed after the firstmask application of FIG. 2C;

FIG. 2E is a cross sectional view in accordance with the teachings ofthe present invention of a second mask used upon the patterned firstlayer of FIG. 2D;

FIG. 3A is a planar view of a representative embodiment of a patternedfirst layer in accordance with the teachings of the present invention;

FIG. 3B is a planar view in accordance with the teachings of the presentinvention of a second mask used upon the representative embodiment ofthe patterned first layer of FIG. 3A;

FIG. 3C is a planar view of one embodiment of a resistive heater inaccordance with the teachings of the present invention;

FIG. 4A is a planar view of another representative embodiment of apatterned first layer in accordance with the teachings of the presentinvention;

FIG. 4B is a planar view of another embodiment of a resistive heater inaccordance with the teachings of the present invention;

FIGS. 5A and 5B are planar views of a plurality of cascaded resistiveheaters in accordance with the teachings of the present invention;

FIG. 5C is a planar view of a plurality of cascaded resistive heatersconnected electrically in serial in accordance with the teachings of thepresent invention;

FIGS. 6A and 6B are planar views of a plurality of grouped resistiveheaters in accordance with the teachings of the present invention;

FIG. 6C is a planar view of a plurality of cascaded resistive heatersconnected electrically in parallel in accordance with the teachings ofthe present invention;

FIG. 7 is a block diagram of a system having a thermo optic packagecomprising resistive heaters formed in accordance with the teachings ofthe present invention; and

FIG. 8 is an alternative embodiment of a thermo optic package comprisingresistive heaters formed in accordance with the teachings of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration, specific embodiments inwhich the inventions may be practiced. These embodiments are describedin sufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that process, electrical or mechanical changes may be madewithout departing from the scope of the present invention. As usedherein, the term substrate includes any base semiconductor structure,such as silicon-on-sapphire (SOS) technology, silicon-on-insulator (SOI)technology, thin film transistor (TFT) technology, doped and undopedsemiconductors, epitaxial layers of a silicon supported by a basesemiconductor structure, as well as other semiconductor structures wellknown to one skilled in the art. The following detailed description is,therefore, not to be taken in a limiting sense, and the scope of thepresent invention is defined only by the appended claims and theirequivalents.

The following description and figures use a reference numeral conventionwhere the first digit of the reference numeral corresponds to the figureand the following two digits correspond to like elements throughout thespecification. For example, the grating of a thermo optic device of thepresent invention has a reference number of 200, 300, 400, etc.corresponding to the grating X00 in FIGS. 2, 3, 4, etc., where X is thenumber of the figure in which the reference numeral appears.

A resistive heater for use with a thermo optic device will now bedescribed that, advantageously, has less mask counts than any prior artheaters. With reference to FIG. 2A, a thermo optic grating in accordancewith the present invention is shown generally as 200. The grating 200comprises a cladding 202 and a core layer 208. It is formed on asubstrate 210 preferably formed of silicon. The substrate, however,could be any variety of well known materials for supporting a thermooptic grating.

The cladding 202 includes an upper cladding 204 and a lower cladding 206that define the shape of the core layer 208. The materials selected forthe cladding 202 and core layer 208 are selected in such a way that theyhave different indices of refraction, either higher or lower than oneanother. As is known, the core layer acts to propagate light byreflecting light at the boundaries between the core layer and thecladding. In a preferred embodiment, the upper and lower claddings 204,206 are formed of silicon oxides while the core layer is formed of asilicon oxynitride.

The core layer, in one embodiment, forms an optical waveguide. It couldbe similar in shape to the Y-shaped optical waveguide with an input andtwo output waveguides as shown in FIG. 1A. It could also be an X-shapedwaveguide, a continually shaped section of waveguide, or other waveguidestructure now known or hereinafter developed.

In one embodiment, the grating is formed by depositing the lowercladding, depositing the core layer, photo patterning the core layer,etching the core layer and depositing the upper cladding. In anotherembodiment, the lower cladding 206 is a grown layer with the core layerand upper cladding being formed in the same manner.

Preferred deposition techniques for these, and after described layersinclude, but are not limited to, any variety of chemical vapordepositions (CVD), physical vapor depositions (PVD), epitaxy,evaporation, sputtering or other similarly known techniques. PreferredCVD techniques include low pressure (LP) ones, but could also beatmospheric pressure (AP), plasma enhanced (PE), high density plasma(HDP) or other. Preferred etching techniques include, but are notlimited to, any variety of wet or dry etches, reactive ion etches, etc.

In the following figures, a resistive heater will be described that isformed on a surface 212 of the grating 200. It will be appreciated thatthis surface 212 is the top of the upper cladding 204.

In a first step after formation of the grating 200, a first layer 220 isdeposited on surface 212 as shown in FIG. 2B. Preferably, the firstlayer is a poly silicon layer. Even more preferably, it is a polysilicon layer doped with an impurity such as arsenic, phosphorous orboron. In one embodiment, the first layer is a phosphorous doped polysilicon having a resistance of about 40 Ω/cm². The first layer, however,only needs to be a material that has a relatively high resistance. Aswill be described later, the first layer will form a heater regionportion of the resistive heater and needs to resist current flow andproduce heat. Accordingly, the first layer could be any variety of otherrelatively high resistance materials, such as representative chromium,indium or other silicon arranged materials.

In one embodiment, the thickness of the first layer is deposited to athickness of at least about 100 Å thick. In another embodiment, thethickness is about 4000 Å of the phosphorous doped poly silicon. Thethickness is primarily dictated according to the heating requirements ofthe thermo optic device. Thicker depositions provide more heatingcapabilities while thinner depositions provide less. The depositiontechnique for the first layer is LPCVD but could be any of the otherforegoing described techniques.

With reference to FIG. 2C, the first layer 220 is acted upon in aphotomasking step. The first layer is masked with a first mask 224 (maskcount 1) and a portion thereof is photo impacted 226. In thisembodiment, the photo impacting is accomplished with an ultravioletlight from a photolithography device well known to those skilled in theart. The photo impacting, however, should not be limited to such anembodiment and may alternatively include X-rays or other light sources.

Thereafter, the photo impacted first layer is etched so that a patternedfirst layer 230 on surface 212 of the grating remains as shown in FIG.2D. In one embodiment, the etch is a reactive ion etch with a plasmacontaining fluorinated chlorine. The etch, however, could be any wellknown etch described above or any etch hereinafter developed.

As depicted, it will be appreciated that the patterned first layer 230is a photomasking island, but could have been produced as a photomaskinghole. The particular embodiment, island or hole, depends upon whetherthe configuration of the first mask is a clear-field or dark-field maskas those terms as well understood in the art. In either event, bothembodiments are embraced by the scope of this invention.

With reference to FIG. 3A, a representative patterned first layer 330 isshown in a top-down planar view as it is formed on surface 312 ofgrating 300 after the process step shown in FIG. 2D. In this embodiment,the patterned first layer comprises a heater region 331 and a conductivepath region 333 on either sides thereof.

In the embodiment shown, the heater region 331 has planar dimensionsLh×Wh, with L being length, W being width, and h being the heaterregion. These dimensions are of no particular size provided they fitwithin the geographic confines of surface 312 of grating 300. The sizeis dictated by how much heat is required by the thermo optic deviceduring use.

In a similar manner, the dimensions L_(C1)×W_(C) and L_(C2)×W_(C), wheresubscript C is the conductive path region and subscript 1 is the leftand 2 is the right conductive path regions, respectively, are not of aparticular size. In fact, the dimensions of both conductive path regionscould be the same or very dissimilar to one another or the heaterregion. The size of the conductive path regions is a function of designaccording to many parameters such as where the heater region is to beconnected to an external control system for the delivery of current, howmuch current is to flow, etc.

The shapes of both the heater region and the conductive path regions,while shown as generally rectangular, could be circular, serpentine,polygonal, triangular, square, or any other geometric shape(s) thatcould be fabricated with the first mask. As is taught herein, the sizeand dimension is a function of, among other things, how much heat isrequired to be generated by the heater region.

Even further, the planar (x-y plane) positioning of the patterned firstlayer, including the heater region and conductive path regions, as it isformed on surface 312 is similarly a function of the thermo optic deviceand in what application the thermo optic device will be used.

Accordingly, another representative embodiment of a patterned firstlayer 430 with a heater region 431 and conductive path regions 433 on asurface 412 of a grating 400 is shown in FIG. 4A. In this Figure, theheater region is formed between the conductive path regions with neckdown areas 441 to depict a much smaller heater region in comparison tothe conductive path regions. Bond pad regions 437 are also formed to,ultimately, facilitate electrical connection the heater region 431.Again, no particular, size, shape, positioning of the patterned firstlayer is required and all embodiments are embraced herein.

In all figures, it should be appreciated that both the heater andconductive path regions are formed of the relatively high resistancematerial in a first mask count even though the heater and conductivepath regions have different functions as will be described subsequently.

With reference to FIG. 2E, in another photomasking step, a second mask232 (mask count 2) is arranged with respect to the patterned first layer230 to isolate the heater region from the conductive path regions. InFIG. 3B, the second mask 332 is exaggeratedly shown over heater region331 to isolate the heater region from each of the conductive pathregions 333 on either side thereof. Like the first mask, the second maskcan be configured as a clear-field or dark-field mask to isolate theheater region from the conductive path region and both embodiments areembraced herein.

Thereafter, the areas of the conductive path regions are plated with arelatively low resistance material to form conductors 235 as shown inFIG. 2E.

In a preferred embodiment, the conductive path regions are electrolessplated with nickel by submergence of at least the conductive pathregions in a liquid bath. In another embodiment, the conductors 235 areformed with any of tungsten, titanium, tantalum, molybdenum or cobalt.In still another embodiment, multiple layer stacks such as TiSix/TiN/Wmay be used to reduce contact resistance between conductor and heaterlayers. The material selected for the conductors, it should beappreciated, need only be a material having a relatively low resistanceso that current can flow relatively well. As such, still other metals,combinations of metals or other materials can be used in forming theconductors.

With reference to FIG. 3C, a representative resistive heater inaccordance with the present invention is shown generally as 340. Itcomprises a heater region 331 and conductors 335 formed over theconductive path regions. The resistive heater is formed on surface 312of grating 300 of a thermo optic device.

In FIG. 4B, the representative resistive heater of the present inventionis shown generally as 440. It includes a heater region 431 andconductors 435 formed over conductive path regions 433 (FIG. 4A). Thebond pad regions 437, after plating, form bond pads 439. As with theother embodiments, the resistive heater 440 is formed on surface 412 ofgrating 400 of a thermo optic device, which in turn, is formed on asubstrate.

It will be appreciated that the foregoing described resistive heater isformed with two mask counts which is at least one less mask count thanany known prior heaters. Advantageously, the practice of this inventionyields quicker formed resistive heaters for use with thermo opticdevices. Since photomasking machinery, and therefore masking operations,is extremely expensive in relation to other types of process machines,such as CVD machines, the practicing of the present invention will alsoresult in financial savings.

During use, the resistive heaters of the present invention are connectedto a control element (perhaps via the bond pads) via the conductors todeliver current to the resistive heater to heat the thermo optic devicethereby changing an optical characteristic thereof. It will beappreciated that the conductors will flow current because they areformed of low resistance material while the heater region will inhibitcurrent flow because it is formed of a high resistance material. Theheater region, in turn, will heat up the thermo optic grating while theconductors will not.

During use, the resistive heaters of the present invention may be usedwith thermo optic devices in a variety of applications. For example, thethermo optic device may be an amplifier, an optical waveguide, a switch,a modulator, or any other optical transmission device.

Heretofore, the resistive heaters of the present invention have beenshown as singular resistive heater on a grating. The present inventionalso embraces pluralities of resistive heaters depending uponapplication requirements for heating a thermo optic device. For example,a plurality of resistive heaters 540 may be cascaded together across asurface 512 of grating 500 as shown in FIGS. 5A and 5B. In FIG. 5C, thecascaded resistive heaters are electrically connected in series acrosssurface 512 of grating 500. In particular, a plurality of heater regions531 are serially interconnected with a plurality of conductive pathregions 530 on either side of each heater region. In this manner, asingle control element may be connected to terminal ends 571 of theconductive path regions and thereby flow a single current through theplurality of heater regions 531 to heat them simultaneously. It will beappreciated that the heater regions, while shown as similar devices, mayall individually have unique shapes and sizes thereby being capable ofdelivering various dissimilar regions of heating to the thermo opticdevice during use.

Alternatively, the resistive heaters of the present invention may bearranged as rows or groupings of resistive heaters 640 in rows A or B onsurface 612 of grating 600 in FIG. 6. In FIG. 6C, the row A of resistiveheaters are electrically connected in parallel. In particular, aplurality of heater regions 631 each having a conductive path region 630on either side thereof is electrically connected in parallel via aplurality of inter-bridging conductive paths 673 having substantiallythe same electrical resistance as the conductive path regions. In thismanner, a single control element may be connected to terminal ends 671of the conductive path regions and thereby flow a plurality of currentsthrough the plurality of heater regions 631 to heat them simultaneously.It will be appreciated that the heater regions, while shown as similardevices, may all individually have unique shapes and sizes thereby beingcapable of delivering various dissimilar regions of heating to thethermo optic device during use.

Those skilled can envision still other arrangements of resistive heatersformed in accordance with the teachings of this invention withoutdeparting from the spirit or scope of the defined claims.

It will be appreciated that during use, the resistive heaters of thepresent invention may be used with thermo optic devices in a variety ofapplications. For example, the thermo optic device may be an amplifier,modulator, gate, filter, time delay element, switch, multiplexer, orother.

With reference to FIG. 7, a system, having as part thereof a resistiveheater formed in accordance with the teachings of the present invention,is shown generally as 741. The system may be an exclusively fiber opticsystem or may be a system having other software and hardware devices, asindicated by the dashed line 745, operably coupled to at least one fiberoptic component thereof.

In either system, a light source 743 will be provided as the source forpropagating light signals along at least one fiber optic line 747. Wellknown light sources include, but are not limited to, laser lightsources. In the embodiment shown, the system 741 includes a plurality offiber optic lines 747.

Coupleable to the fiber optic lines via a plurality of input fiber opticports 751 is a thermo optic package 749. Contained within the thermooptic package is at least one thermo optic device 753 having at leastone resistive heater formed in accordance with the present invention. Inthe embodiment shown, the thermo optic device 753 is coupled to theinput fiber optic port 751 via an input connector 755 while an outputconnector 757 couples the thermo optic device to an output fiber opticport 759. In turn, the output fiber optic port 759 is coupled to anotherfiber optic line 747 of system 741.

During use, a system user merely needs to couple fiber optic lines 747to the input and output fiber optic ports of the package 749 to readilyachieve a thermo optic device having the advantages offered by theresistive heaters of the present invention.

With reference to FIG. 8, an alternative embodiment of a thermo opticpackage 849 is shown having a thermo optic device 853 with a singleinput connector 855 and a plurality of output connectors 857. The inputconnector 855 connects with input fiber optic port 851 which is readilymatable with a fiber optic line 847 of a system. The output connectors857 of thermo optic device 853 are each matable with an output fiberoptic port 859.

In another embodiment, the single input connector of the thermo opticdevice 853, having a resistive heater formed in accordance with thepresent invention, may alternatively be replaced with two or more inputconnectors while the two output connectors may be replaced with one ormore output connectors depending upon the type and desired use of thethermo optic device 853.

Conclusion

The above structures and fabrication methods have been described, by wayof example, and not by way of limitation, with respect to resistiveheaters for thermo optic devices.

In particular, resistive heaters formed in two mask counts on a surfaceof a grating of a thermo optic device have been described that eliminateat least one mask count from prior art manufacturing methods. Theresistive heater is comprised of a heater region and a conductive pathregion formed together in a first mask count from a relatively highresistance material. A conductor formed from a relatively low resistancematerial is formed directly on the conductive path region in a secondmask count. Thermo optic devices formed by these two mask count methodsare also described.

As a result, resistive heaters of this invention can be formed quickerand cheaper without any corresponding sacrifice in quality, reliabilityor longevity.

The present invention has been particularly shown and described withrespect to certain preferred embodiment(s). However, it will be readilyapparent to those of ordinary skill in the art that a wide variety ofalternate embodiments, adaptations or variations of the preferredembodiment(s), and/or equivalent embodiments may be made withoutdeparting from the intended scope of the present invention as set forthin the appended claims. Accordingly, the present invention is notlimited except as by the appended claims.

1. A method, comprising: providing a grating having a surface; forming apatterned polysilicon first layer on the surface with a first mask; andforming a resistive heater on the surface from the patterned first layerwith a second mask.
 2. The method according to claim 1, wherein theforming the patterned first layer further includes depositing a firstlayer on the surface.
 3. The method according to claim 2, wherein theforming the patterned first layer further includes photo impacting thefirst layer.
 4. The method according to claim 1, wherein the forming theresistive heater further includes plating a conductive path region ofthe patterned first layer.
 5. The method according to claim 1, whereinthe forming the resistive heater further includes masking a heaterregion of the patterned first layer.
 6. The method according to claim 1,further including forming the grating on a substrate.
 7. An apparatusformed in accordance with the method of claim
 1. 8. A method,comprising: providing a grating having a surface; forming a polysiliconheater region and a conductive path region on the surface with a firstmask; and masking the heater region with a second mask to form aresistive heater on the surface.
 9. The method according to claim 8,wherein the forming the heater region and the conductive path regionincludes depositing a first layer on the surface.
 10. The methodaccording to claim 8, wherein the masking the heater region with thesecond mask to form the resistive heater further includes plating theconductive path region.
 11. The method according to claim 8, furtherincluding forming the grating on a substrate.
 12. An apparatus formed inaccordance with the method of claim
 8. 13. The method of claim 8,wherein forming the heater region includes forming the heater region toa thickness of about 100 Angstroms.
 14. The method of claim 8, whereinforming the heater region includes depositing polysilicon.
 15. A method,comprising: providing a grating having a surface; forming a patternedpolysilicon first layer on the surface with a first mask; and forming aneck down, resistive heater on the surface from the patterned firstlayer with a second mask.
 16. The method according to claim 15, whereinthe forming the patterned first layer further includes depositing afirst layer on the surface, and photo impacting the first layer.
 17. Themethod according to claim 16, wherein the forming the resistive heaterfurther includes plating a conductive path region of the patterned firstlayer.
 18. The method according to claim 17, wherein the forming theresistive heater further includes masking a heater region of thepatterned first layer to a smaller dimension than the conductive pathregion.
 19. The method according to claim 15, wherein providing thegrating includes forming a lower cladding on a substrate and forming anupper cladding on the lower cladding, wherein the upper cladding andlower cladding have different indexes of refraction.
 20. The methodaccording to claim 19, wherein forming a lower cladding includes growingthe lower cladding on a substrate.
 21. A method, comprising: providing agrating having a surface; forming a patterned, polysilicon\metal firstlayer on the surface with a first mask; and forming a resistive heateron the surface from the patterned first layer with a second mask. 22.The method of claim 21, wherein the first layer includes one ofchromium, indium and silicon.